Usage Note: The prefix bi-, when modifying an adverb of time, means that the time interval should be doubled. Thus a bimonthly meeting takes place once every two months, and a biweekly meeting once every two weeks. The prefix semi-, in the same context, means that the interval of time should be halved. Thus, a semimonthly meeting is held twice a month (that is, every half month), and a semiweekly meeting twice a week. Words with the prefix bi- and semi- are often confused, so expressions such as every two months or twice a month are often preferable in order to avoid misinterpretation.

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usage: Most words referring to periods of time and prefixed by bi- are potentially ambiguous. Since bi- can be taken to mean either “twice each” or “every two,” a word like biweekly can be understood as “twice each week” or “every two weeks.” Confusion is often avoided by using the prefix semi- meaning “twice each” (semiweekly; semimonthly; semiannual) or by using the appropriate phrases: twice a week; twice each month; every two months; every two years.

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